A milliohmmeter is a precision instrument for measuring the variety of the smallest resistance values in different circuits. The milliohmmeter takes its name from the Ohm, the unit for electrical resistance. Milliohmmeters are used instead of conventional ohmmeters where they are required to measure resistance limits at the lower end of the scale, of only a few ohms. At this level the intrinsic resistance of the ohmmeter , the leads and contacts, can distort the readings and affect the results. For precise measurement of such small resistances only a milliohm meter can be used. The modern meters usually have both digital read outs as well as LCD displays. Micro-ohm meters can be included in the measurement range if they have a high degree of accuracy. The measuring of milliohmmeters generally covers micro-ohms to megohms. In contrast to the milliohm meter there are mega-ohm meters which measure ranges in the region of kilo-ohms to peta-ohms.
The milliohm meter is commonly used for monitoring production processes and for quality assurance. For detailed measurements relay coils, generators, switches, connectors, transformers, parallel and secondary circuits and platinum wires are removed. Many of the available milliohmmeters on the market have an RS232 interface connection for transferring measurements to computers and printers. The computers require additional software to read the data. As well as reading very low resistance levels, milliohmmeters must also be able to record these over a very short period of time, sometimes less than 100 milliseconds. Additionally, some milliohm meters have a comparator function, which can detect mean deviations and when parameters are exceeded. Other milliohmmeters can also measure voltages as frequencies. The exterior casing of many milliohm meters has been designed to be splash-proof, which extends their uses to include, ships, planes and mines.
The principle of a milliohmmeter is to use the way that a transistor reasons, to measure cables and contacts. With a constant current flow, i.e. a constant current and an upstream digital voltmeter, this will provide a simple but effective milliohmmeter. Milliohmmeters are usually constructed as a four point measuring device. This gets around the problems caused by the intrinsic resistance of power cables and contacts in a two point measurement, by avoiding, or adjusting for errors. As the voltage drops at the resistor the current can be tapped and fed into a voltmeter. Then, by knowing the current and measuring the voltage at the voltmeter the resistance (R) can be calculated using Ohm’s Law as the voltage (U) divided by the current (I). The secret of the four point technique is to use separate paths along which to measure the current and the voltage drop.« Zurück